A bipolar secondary battery such as a lithium-ion battery used in an electrically-driven vehicle such as a hybrid electric vehicle (HEV) or an electric vehicle (EV) comprises a laminated body of cells. Each cell comprises an electrolyte layer, a positive electrode active material layer in contact with one surface of the electrolyte layer and a negative electrode active material layer in contact with another surface of the electrolyte layer.
A plate-like current collector having a larger area is laminated on an outer side of each of the positive electrode active material layer and the negative electrode active material layer. A fluid sealant is filled between the current collectors corresponding to the outer periphery of the electrolyte layer to prevent the electrolyte layer from being degraded due to moisture in the air. The electrolyte layer, the positive electrode active material layer and the negative electrode active material layer are thereby blocked from outside air.
A fluid sealant such as paraffin undergoes electrolysis under a high voltage. The electrolyzed fluid sealant can no longer keep a necessary insulation property.
JP2009-252548A published by the Japan Patent Office in 2009 proposes to divide the fluid sealant into a plurality of sealed layers such that a fluid sealant does not undergo a high voltage.